Attending to the watering needs of plants has been an issue for mankind ever since the cultivation of plants began many thousands of years ago. The present invention provides several methods for determining when plants require watering, and methods of attending to the watering of plants including signaling the grower that the plants are in need of hydration.
Traditionally, the overall thickness of plant leaves has mostly been determined from dissected leaves under microscopes showing leaf cross-sections and including scale bars in the field of view. Although mechanical micrometers or calibers could be used for measuring the thickness of leaves, the use of mechanical micrometers or calibers is cumbersome and time consuming. In addition, the use of mechanical micrometers or calibers may damage leaf cells if the arms of the device used are closed on the leaf under investigation too forcefully during measurements.
In order to overcome the obstacles named above for measuring the overall thickness of leaves more easily and in vivo while preventing the leaf from damage, a sensor system was developed utilizing a Hall-effect sensor integrated circuit (IC), a magnet, and a gripper like fixture, which shall be denoted “Leaf Thickness Meter Utilizing a Hall-Effect Sensor” (LTMH).
It is well known that the spectral distribution of light reflected from a plant leaf differs from that of an incident beam. These spectral specifics of reflected light indicate the presence and amount of absorbers inside the leaf. The same is true for light transmitted through a plant leaf. Since leaf cells are mainly comprised of water and pigments (such as chlorophyll and carotenoids), the spectrum of light reflected from a leaf is determined to a large extent by absorption characteristics of water and pigments. In the near infrared (NIR) range, absorption by pigments becomes negligible and absorption by water becomes dominant.
The pressure that develops in leaf cells of plants due to the high elastic modulus of the walls of leaf cells and due to the presence of solutes inside leaf cells is called turgor pressure. In non-stressful situations in terms of water supply, i.e. when plants are not limited in the uptake of water, they typically regulate the turgor pressure of their leaf cells to be high. Turgor pressure in leaf cells can achieve several Mega-Pascals (MPa). Under high turgor pressure any minute change of the water content of leaf cells results in large fluctuations of turgor pressure of these cells. In fact, several studies have shown that a loss of the relative water content (RWC) of leaf cells of just 15% from their nominal high RWC-values may cause the turgor pressure of these leaf cells to decline substantially, or to be completely lost. Thus, turgor pressure of leaf cells is one of the most sensitive parameters to detect the onset of leaf dehydration. If only slight amounts of water move out of leaf cells due to the development of water deficit stress (WDS), turgor pressure typically decreases substantially in response.
If an unusual decrease of turgor pressure in leaf cells could be detected non-invasively and in real-time, such a detection could potentially signal the onset of leaf dehydration and hence the development of WDS in plants.
Traditionally, turgor pressure of cells has been determined by either one of two ways. Turgor pressure of cells has been measured by some studies directly by actually inserting microcapillary tubes into the cells under test and measuring the pressure inside the cells directly using pressure transducers or micromanometers. Clearly, such an approach is destructive, time-consuming, rather cumbersome to conduct, and mainly of academic interest. The method mostly used in practical applications determines the turgor pressure of leaf cells indirectly by determining the overall water potential (Xw) and the solute water potential (Xs) of a leaf under test by appropriate means. Since the overall water potential of leaf cells equals the sum of the solute water potential and the pressure water potential (Xp; which is the turgor pressure) of leaf cells:Xw=Xs+Xp one can easily solve the equation for the turgor pressure (Xp) and determine the turgor pressure once the overall water potential and the solute water potential of leaf cells are known. Various techniques for determining the overall water potential and the solute water potential of leaf cells have been described. However, all of the techniques for determining the overall water potential and the solute water potential of leaf cells are destructive as well, since all of those techniques require the taking of leaf samples for examination and all of those techniques eventually destroy those leaf samples. Furthermore, all of those techniques require the use of laboratory equipment, such as precision scales, pressure chambers, microscopes, and the like, and the determination of turgor pressure of leaf cells may take several hours or several days in order to receive final data using those methods. Thus, all of the traditional methods for the determination of turgor pressure of leaf cells may be considered destructive, time consuming, and rather difficult to conduct, and can therefore not be used for the detection of WDS in plants by monitoring turgor pressure in a leaf cell non-destructively and in real time.
The method described here overcomes the obstacles of traditional methods for determining the turgor pressure of leaf cells and measures the turgor pressure of leaf cells non-destructively, conveniently, and in real time. For doing so, the method described here utilizes the principle of “pressure broadening and shifting of spectral absorption bands”.